1. Field of the Invention
This invention relates to a method and apparatus for collecting and analyzing Raman scattered light. The apparatus includes an atomic vapor cell configured to spectrally disperse the light by resonant dispersion while simultaneously suppressing the Rayleigh Scattering and other background scattering through resonant absorption.
2. Related Art
Raman spectroscopy has long been a primary tool for molecular diagnostics. The Raman mechanism is based on light scattering from molecules in a sample volume and is usually done using a narrow linewidth laser as a source. When a sample is illuminated, the light that scatters from it echoes the character of the molecules in the sample volume. If the source has a narrow frequency band, then one can distinguish several molecular scattering mechanisms by the spectrum of the light that is collected. Assuming that the illumination frequency is far away from a resonant absorption feature associated with any of the molecules in the sample volume, then there are three primary components of the scattered light. The strongest component is the elastic scattering from the molecules. This light is usually termed Rayleigh Scattering, and it is only very slightly shifted in frequency from the source frequency due to the Doppler effect associated with the translational motion of the molecules. A second component arises from scattering off of the acoustic waves in the medium and is frequency shifted by the Doppler shift associated with the speed of sound. The second component is often call Brillouin Scattering, and it appears most notable in high pressure or low temperature gases where the mean free path of the molecules is small compared to the relevant acoustic wavelength. The third component, Raman scattering, is due to inelastic scattering from the molecules themselves and is associated with the internal modes of molecular energy. Since the internal modes of energy are unique to each molecular species, this type of scattering is exceptionally useful. The presence of specific spectral features indicates the presence of particular molecules in the sample volume, and the strength of the features can be interpreted to give the concentration of those molecules and their temperatures. This means that Raman spectroscopy can be used to identify what molecular species are present within a sample volume and the temperature of those species. In certain cases, the molecules may not be in thermal equilibrium. Under these circumstances, the spectral features and their relative strengths can give the nonequilibrium state of the molecule, yielding such information as the translational, rotational and vibrational temperatures. Thus, Raman spectroscopy may also be used to produce images of complex environments such as combusting gases, mixing gas streams, etc.
The Raman Spectrum is separated into two regions, one associated with scattering from purely rotational transitions, called Rotational Raman Scattering, and one associated with scattering from vibrational-rotational transitions, called Vibrational Raman Scattering. These two regions have different features. The, Vibrational Raman scattering has features associated with each molecular species clustered together at various locations in the spectrum, whereas the rotational Raman spectrum has all the Raman active species interleaved within it. In additional, since the rotational modes of a molecule have significantly lower energy than the vibrational modes, the Raman shift associated with rotational motion is rather small, on the order of a few wavenumbers. The vibrational shifts, on the other hand, are hundreds to thousands of wavenumbers. This means that the strong Rayleigh line is very close to the rotational spectrum and in most cases, obscures it. Thus, even though rotational Raman scattering is usually on the order of a factor often stronger than vibrational Raman, it is not often used for spectral analysis.
Much prior work has been done to develop various Raman devices that can effectively collect and analyze the Raman spectrum. This has been difficult because Raman scattering is exceptionally weak and occurs in the presence of strong Rayleigh scattering plus background from the sample cell walls and, in some cases, fluorescence. The typical Raman set-up consists of a light collection system which images the scattering onto a narrow slit, through which the light passes into a spectrometer, is dispersed by a grating and then passes out of a second slit onto a detector. In some cases the second slit is removed, and the detector is replaced by a camera. The scattering spectrum is spread out in space by diffraction from the grating. The grating is turned so the Raman line of interest passes through the slit onto the detector or the Raman spectrum is imaged by the camera, and the strong spectral background features are not seen by the camera or the detector. Often this arrangement does not give enough extinction to the other light components, and a second spectrometer stage must be added.
Various approaches to improving this set-up have been proposed.
Barrett, U.S. Pat. No. 3,909,132, proposed using an interference filter to transmit the rotational lines in order to measure temperature. This concept matches the approximately equal spacing of the rotational lines from one another to the regularly spaced transmission frequencies of an interferometric filter and recognizes that the progressive mismatch at higher energy may be used to determine the temperature.
Other attempts in the past include:
Stamm, U.S. Pat. No. 2,483,244, recognizes the need to provide a device of high dispersion to separate light of closely adjacent wavelengths to the maximum degree, such as found in Raman spectra. To accomplish this, the spectrometer system taught uses a Wernicke prism that includes in combination the use of a central liquid prism sandwiched between two end prisms of crown glass. The glass and prisms are symmetrically arranged and have substantially identical optical properties, with the central liquid section prism being filled with a clear ester or an acid. Various acids are disclosed in this reference.
Cary, U.S. Pat. No. 2,940,355, discloses two monochromator sections which receive monochomatic light from a source for permitting illumination of a sample over a greatly extended range relative to the use of a single monochromator, for reducing the intensity of background radiation relative to the intensity of the Raman light scattering or spectra-lines to be measured. The scattered light or Raman spectrum from the double monochromator is amplified and recorded.
Tochigi, et al., U.S. Pat. No. 4,586,819, discloses a Raman-scattered light is separated from the reflector laser beam by a filter, and the extracted Raman-scattered light is then passed through a single-monochromator for analyzing a sample. In the example given, the sample can be foreign matter of one micron or less in diameter on an integrated circuit wafer. The filters utilized, are indicated as being commercially available filters such as those that consist of dielectric-coated glass filters, that are selected for transmitting a predetermined wavelength band of light. The filters are used for separating the wavelength bands of the laser beam into spectra in shorter and longer wavelength regions. By comparing the spectra in these wavelength regions, the temperature of the sample under analysis can be exactly detected.
Webster, U.S. Pat. No. 4,684,258 teaches the reduction of interference fringes in a light beam passed through a passive cavity by including a Brewster-plate spoiler in the cavity, and oscillating it back-and-forth over an angle for causing the cavity resonances to tune in a frequency over a range that is a multiple of the period of the interference fringes. A diode laser is used to direct a beam of coherent light through a lens into a prism-like cell, from which cell the beam is directed to a Brewster-plate spoiler.
Wada, U.S. Pat. No. 5,217,306, discloses the passage of light through an optical filter located in a target region where temperature is to be measured. Pulsed laser light is passed through the optical fiber resulting in Raman-scattering. The Raman-scattering light is detected for computing the temperature of the region in which the optical fiber is located. An optical filter and a branching filter are used in the system. There is no teaching of what the optical filter consists of. The light branching filter is indicated as being made up of a diffraction grading or a thin-film optical filter such as a multilayered dielectric film.
Alfano, et al., U.S. Pat. No. 5,261,410, provides for irradiating human tissue with a beam of infrared monochromatic light, extracting the infrared Raman spectrum from the tissue, and comparing the obtained Raman spectra with standard infrared Raman spectra established as a standard for malignant tumor tissue in one case, and benign tumor tissue in another case, for making a determination as to whether the tissue under study is either benign or normal, or malignant. The system uses a fiber optic cable for transmitting laser light onto as tissue under examination, and thereafter for picking up the Raman-scattered light, and passing it through an interferometer, detector, signal processor, computer, and ratio meter for computerized comparison of the standards obtained from prior tissue studies, for thereafter displaying the results.
Maguire, U.S. Pat. No. 5,262,644, uses a fiber optic cable for irradiating a sample with coherent light from an infrared laser, and another fiber optic cable for detecting Raman-and Brillouin-scattered light for analysis.
Sai, et al., U.S. Pat. No. 5,449,233 and U.S. Pat. No. 5,618,108 describe an improvement over prior Optical Time Domain Reflectometry (OTDR) devices for measuring temperature by detecting and processing backward Raman-scattered light along points in the optical fiber. As shown in FIG. 1, the prior art system includes a light source that emits pulsed-light into a directional coupler, and therefrom into an optical filter probe used to measure temperature. The probe is arranged about an object who temperature is to be measured. The pulsed-light traveling in the fiber optic probe is Raman-scattered therein, whereby backward scattered light is returned from the fiber to the directional coupler. The Raman-backward scattered light is directed via a directional coupler into an optical filter, the latter including two filters for extracting anti-Stokes' light and Stokes' light from the backward Raman-scattered light, respectively. Filters having passbands for the wavelength of the anti-Stokes' light and the Stokes' light are used to individually extract these wavelengths of lights. As further shown, analog-to-digital filters are then use to sample and digitize the two light signals, for processing by signal processor to calculate a temperature distribution derived from the ratio of the two digitized backward Raman-scattered light signals. The improvement made in the system overcomes problems in the prior art partly by attenuating the intensity of the Stokes' light to be nearly equal to the intensity of the anti-Stokes' light. Switching means are used to select one of the two light signals, for passage through an analog-to-digital converter, and signal processing for calculating a required temperature distribution along an optical fiber.
Obremeski, et al., U.S. Pat. No. 5,498,875 discloses a system in which the analyte content of a sample is determined by passing two light signals of different wavelengths (differ by at least three nanometers) through a sample for generating two output signals. One of the output signals is a resonant signal having a wavelength independent of the associated input signal. The other output signal is a non-resonant signal that has a peak wavelength dependent upon the wavelength of the associated input light signal. The two signals are then processed for determining the difference therebetween for providing data about the analyte content of the sample. Liquid crystal filters are used in the system to provide the modulator. In one embodiment of the invention Raman-scattered light signals are processed. In another embodiment, Rayleigh scattered light signals are processed.
Carlsen, et al., U.S. Pat. No. 5,506,678 provides for transmitting a laser beam transmitted into an elongated tubular container for a gaseous substance that is to be analyzed. The gas molecules cause a portion of the light to be Raman-scattered. The tubular enclosure for the gas sample is made highly reflective to the Raman-scattered light for increasing the intensity of the Raman-scattered light outputted from an exit slit in the tubular member to collection optics. The scattered light from the collection optic is passed through a filter, a focusing lens, and therefrom to a detector for transforming the light signals into electrical signals for processing by a signal processor.
Alsmeyer, et al., U.S. Pat. No. 5,638,172, provides for irradiating both a reference material and a chemical composition with monochromatic radiation, and thereafter detecting and processing the Raman-scattered light therefrom. Comparisons are made between the Raman spectrum from the standard relative to the Raman Spectrum obtained from the chemical sample, for analyzing the composition of the chemical sample.
Sai, U.S. Pat. No. 5,639,162, teaches pulsed-monochromatic light passed through a fiber optic temperature transducer, and the back-scattered Raman light detected and processed for determining the temperature. An optical filter is used for filtering the back-scattered Raman light and separating it into the anti-Stokes' component and the Stokes' component, for further processing.
None of these efforts, taken either alone or in combination, teach or suggest all of the benefits and utility of the present invention.